Thermostable coating materials

ABSTRACT

Novel intermediate compounds capable of being applied to a semiconductor precursor by spin-on methods which exhibit good planarity and gap-fill characteristics, the cured composites of which are capable of withstanding temperatures in excess of 500 DEG  C. The disclosure further encompasses a process for fabricating semiconductor devices utilizing the intermediate compounds and its composite.

This application is a division, of application Ser. No. 08/250,224 filedMay 27, 1994, now U.S. Pat. No. 5,145,655.

TECHNICAL FIELD

The invention relates generally to novel thermostable composites andrelated intermediates as well is methods for use of the same, moreparticularly to the use of these novel thermostable composites andintermediates in the fabrication of semiconductor devices. Thecomposites comprise perylene diimide and a silsesquioxane and are madefrom perylene anhydride and an aminosilane.

BACKGROUND

The fabrication of integrated circuits depends upon the construction ofa desired pattern of electrically active impurities within asemiconductor body, and upon the formation of a correspondinginterconnection pattern for their operating characteristics.

Fabrication of integrated circuits thus involves a great number ofdifferent processes well known in the art, examples being chemical vapordeposition of semiconductors and insulators, oxidation, solid statediffusion, ion implantation, vacuum deposition, various lithographictechniques and numerous types of etching techniques. A typical ICfabrication process utilizes a great number of cycles, each of which mayutilize a specific sequence of one or more of the above referencedfabrication techniques.

As is well known in the art, films applied to the semiconductor may beused to selectively limit the effect of a certain process to aregionally specific area. When a film is used in this manner it iscommonly referred to as being a mask. For example, in order to dope aregionally specific area on a semiconductor one may apply a film that isimpermeable to the specific doping process to be utilized. After dopingthe desired region, the film may be removed.

It is necessary for the various films utilized in the IC fabricationprocess to be compatible with a large number of lithography, etching,doping and other IC fabrication techniques. However, the polyimides orspin-on glasses (SOG) commonly used as films today have a practicaloperating limit below 500°C. This severely restricts the sequence offabrication steps that can be employed in the presence of such films.

Therefore there exists a need for a film capable of application onsemiconductor devices by methods known in the IC fabrication art that iscapable of withstanding temperatures in excess of 500° C. There furtherexists a need for such a film that is compatible with a wide range of ICfabrication processes such as various etching, doping, deposition andimplantation techniques.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a thermostable compositecapable of withstanding temperatures in excess of 500° C.

It is a further object of the invention to provide a thermostablecomposite for semiconductor devices which may be applied by methodsamenable to the IC fabrication.

It is a further object to provide a coating having the requisiteplanarity and gap-fill characteristics to be useful in the fabricationof semiconductor devices.

It is a further object of the invention to provide a thermostablecomposite exhibiting RIE etch characteristics that allow it to bevariably etched relative to other materials commonly used in ICfabrication.

It is a further object of the invention to provide a thermostablecomposite capable of acting as a mask and/or mandrel in that it iscapable of being removed by means well known in the art of ICfabrication, examples being CF₄ /oxygen RIE and chemical-mechanicalpolishing.

These and other objects, features and advantages are provided by thepresent invention, which in one aspect relates to a thermostablecomposite comprising: (a) a perylene diimide; and (b) apolysilsesquioxane. In another aspect the thermostable compositecomprising perylene diimide and the polysilsesquioxane are in a ratio ofone perylene for each of four or more silicon atoms in thepolysilsequioxane.

The invention further provides for a thermostable composite produced bythe process of combining perylene dianhydride and four equivalents of aprimary or secondary aminoalkyl alkoxysilane in a solvent, mixing thecombination at a temperature between 15° and 80° for four to twentyhours, setting by heating from the mixing temperature to a finaltemperature of about 150° over the course of 10 to 30 minutes; andcuring at 500° C. under nitrogen for about 30 minutes. The composite maybe further characterized in that it exhibits less than 1% weight lossupon exposure to temperatures from 500° to 620°C. for one hour and alsoexhibits an RIE etch rate more than 4 times that of silicon nitride in20/80 CF₄ /O₂ and less than 1/4 that of silicon nitride in pure O₂.

An additional aspect of the invention includes a process for making athermostable composite which comprises (a) mixing perylene dianhydridein an inert solvent with about four equivalents of one or moreaminosilanes having a formula of: ##STR1## wherein R is selected fromthe group consisting of H, alkyl and alkylamine; X is selected from thegroup consisting of alkyl of at least two carbons and aryl; Z is alkylor a direct bond; Y is selected from the group consisting of alkoxy,halo and silazane; and (b) heating said reactants, thereby forming athermostable coating. For example, the aminosilane utilized may be anaminoalkyltrialkoxysilane such as aminopropyltriethoxysilane.

An additional aspect of the present invention includes a process formaking a thermostable coating comprising the steps of mixing perylenedianhydride in an inert solvent with about 4 equivalents of one or moreaminosilanes having a formula of: ##STR2## wherein R is selected fromthe group consisting of H, alkyl and alkylamine; X is selected from thegroup consisting of alkyl of at least 2 carbons and aryl; Z is alkyl ora direct bond; Y is selected from the group consisting of alkoxy, haloand silazane; and heating said reactants, thereby forming a thermostablecoating.

The above process may also be performed utilizingaminopropyltriethoxysilane. In addition, the process may contain theadditional step of adding from 0.05 to 0.15% by weight of water to theaminosilane 1 to 2 hours before mixing with the perylene dianhydride.The mixture of the perylene dianhydride and aminosilane may also be agedprior to heating. By "aged" is meant that the perylene dianhydride andthe aminosilane are allowed to remain in contact for some period of timeat ambient temperature. Furthermore, in the heating of said reactants,the temperature may be increased from room temperature to approximately150° C. along with the additional step of curing at a temperaturebetween 500° C. and 620° C.

The present invention further encompasses a method of fabricating anintegrated circuit comprising the steps of: providing a substrateincorporating integrated circuit elements; combining perylenedianhydride and an aminoalkylalkoxysilane in a solvent thereby forming amixture; applying said mixture to the substrate; and heating the mixtureto produce a thermostable film on said substrate. The mixture may beapplied by various spin-on techniques. The method may also furthercomprise the additional step of removing at least a portion of saidfilm, an example being by 80:20 oxygen/CF4 RIE or chemical-mechanicalpolishing. In addition, prior to removal of a portion of said film thesemiconductor substrate may be subjected to a temperature exceeding 500°C.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph of the weight of Accuglass SQ-2 as a function of timeand temperature.

FIG. 2 is a graph of the weight of the thermostable composite, formedfrom the reaction between perylene dianhydride andaminopropyltriethoxysilane in NMP, as a function of time andtemperature.

FIG. 3 is a graph depicting the etch rate of multiple compounds undervaried CF₄ /O₂ gas ratios.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a novel thermostable composite which isbelieved to comprise a perylene diimide and an alkylpoly(silsesquioxane) having the formula of: ##STR3##

The thermostable composite shown above may be made as shown by thereaction chemistry set forth below: ##STR4##

The method of making the thermostable composite involves providing asolution or suspension of perylene dianhydride in an appropriatesolvent, such as N-methylpyrrolidinone (NMP). Other useful solventsinclude dimethylacetamide (DMAC), dimethylformamide (DMF),dimethylsulfoxide (DMSO) or suitable mixtures of these solvents withvarious conventional hydrocarbon solvents or other inert solvents inwhich a compound of Formula I is soluble.

An aminoalkyl alkoxysilane (AAS) encompassed by Formula I is preferablyprovided in an amount such that the ratio of the perylene anhydride andthe AAS is approximately in a ratio of 1:4 or more. The AAS is added tothe perylene dianhydride solution or suspension. It is possible toutilize more than one aminoalkyl alkoxysilane together with the perylenedianhydride to form various composites of formula V. The components arepreferably mixed at temperatures in the range of 15° C. to 80° C. andare stirred for 4 to 20 hours to insure uniformity of the solution.Thereafter, the mixture may be heated at temperatures to about 150° C.and then subsequently from 500° C. to 620° C. in an inert atmospherecausing the formation of the composite of Formula V. Generally thetemperature, duration and atmospheric conditions necessary to cure theintermediate will depend upon the thickness of the intermediate, theamount of solvent present with the intermediate and other conditionsknown to those skilled in the art. Typically, a film formed fromaminopropyltriethoxysilane in NMP having a thickness of about 1.5microns will cure in 30 minutes when subjected to a temperature ofapproximately 500° C. in a nitrogen atmosphere.

As indicated above, one of the objects of the present invention is toprovide a thermostable composite which is advantageous in thefabrication of semiconductor devices, such as integrated circuits. Insolution in appropriate solvents, such as NMP, the compound of formulaII compound of formula IV has excellent planarity and gap-fillcharacteristics and it is capable of being applied to a semiconductorsubstrate having integrated circuit elements by techniques well known inthe art, an example being spin-on applications. Formula II is meant toindicate that each anhydride of the perylene dianhydride has been openedby one mole of amine to provide an amide and a second mole of amine hasformed a salt with the liberated carboxylic acid. The substitutionpattern is probably statistically random, but this is not known.

Where the soluble intermediate of formula II is intended to be appliedin solution with spin-on techniques, such as in IC fabrication, thesolvent should be of sufficient volatility to allow significant removalat temperatures about 150°. The solvents are not unduly limited andinclude those enumerated hereinabove.

It has been discovered that in some cases superior spin-on coatings maybe achieved by performing additional steps during the preparation ofcompounds having formula IV, an example being the compounds created fromaminopropyltriethoxysilane in an NMP solvent. For such compounds it isadvantageous to equilibrate the aminoalkyl alkoxysilane with a smallamount of water, from 0.5 to about 1.5 μl H₂ O per ml of alkoxysilane,with about one 1 μl/ml being preferred. Excess water (greater than 2μl/ml) tends to create spin irregularities as well as cause the film tocrack whereas excess of certain aminoalkyl alkoxysilanes, such asaminopropyltriethoxysilane, may cause dewets on apply or pinholes in thecoatings. It is similarly advantageous to age this solution forapproximately one to two hours prior to addition of the AAS to theperylene dianhydride solution or suspension. It has been discovered thataging the aqueous solution appears to produce a film that is less likelyto haze or dewet.

The AAS solution and the perylene dianhydride are mixed forapproximately 4 to 20 hours, preferably for 12 hours using methods wellknown in the art, examples being mechanical stirrers or roller mills.The mixture may be warmed at temperature from 50° C. to 80° C. for asimilar length of time to likewise insure solution uniformity. Forcertain intermediates it may also be preferable that the ratio of thereactants is close to stoichiometric, since excess anhydride appears tofavor hazy films post initial thermal cure.

It is also possible to add a compound capable of forming a surfactantwithin the mixture, such as tetrafluorophthalic anhydride, to improvefilm quality.

The method and manner of applying spin-on solutions, such as those ofFormula IV, to a semiconductor device are well-known in the art and thespecific application of the same will vary with regard to the tolerancesdemanded by the particular semiconductor device, the desired thicknessof the coating and the solution being applied. It has been found thatthe soluble intermediates of Formula IV can be applied as about 41%solutions by volume in NMP at approximately 2500-6000 rpm and spin timesof approximately 15 seconds.

After the solution of Formula IV has been applied to the semiconductoror IC precursor, the solution should be heated in order to drive off thesolvent. The actual heating conditions, i.e. temperature, pressure andtime, may vary over wide ranges depending on the AAS used, the solvent,the thickness of the spin-on coating as well as other factors which areapparent to those skilled in the art. However, the solvent, alcohol andwater (formed as by-products in the formation of amides andpolysilsesquioxanes) will typically be driven off when the coating iscured.

After applying the solution of formula IV to the semiconductor substrateit may be cured by subjecting the coated substrate to heat. It ispreferred that the cure profile proceed from room temperature and beincreased in a ramp in order to reduce the possibility of developingcracks in the film or coating. The spin-on coating may, thus, be heatedfrom approximately 15° C. to a final temperature of about 150° C. overthe course of 10 to 30 minutes. After the ramped heating the coating maythereafter be cured at temperatures from 500°C. to 620° C. in an inertatmosphere, such as nitrogen. Films of formula IV, thermally cured to400° C. still appear bright red; after 600° C. the films of formula Vappear gray. The length of time necessary to cure the coating willdepend upon the temperatures used, the thickness of the coating andother conditions well known to one skilled in the art. For a typicalsemiconductor substrate film of about 1 micron thickness, curing timesof approximately 30 minutes at 500° C. in a nitrogen atmosphere will besufficient to adequately cure the coating, thereby forming thethermostable composite of formula V. It is believed that, in curing, tothe composite of perylene diimide and silsesquioxane is formed. Thereaction is accompanied by pyrrolytic decomposition of the aminoalkylresidue on the silicon. Due to potential outgassing from the compositeit is preferred that the compound of formula IV be cured at atemperature greater than the temperatures involved in later fabricationsteps to which the cured compound of Formula V will be exposed. Forexample, if the cured compound will be exposed to the subsequentdeposition of polysilicon at a temperature of 590° C., then the compoundof formula IV may be cured at a temperature of 620° C.

FIG. 1 represents data obtained from the testing of a spin-on glass(SOG) commonly used in the IC fabrication processes, namely AccuglassSQ-2198 which is commercially available through Allied Signal. This SOGwas subjected to temperatures in excess of 500° C. and as can be seen,it experienced extensive decomposition at temperatures over 500° C.However, by reviewing FIG. 2 it can be seen that the coatings of thepresent invention, here coatings formed in accordance with Example 1below, form a thermostable composite above 500° C. and experiencesignificantly less decomposition at temperatures exceeding 500°C.

This spin-on film may be utilized in connection with application ofmaterials or other processes which subject the semiconductor precursorto temperatures in excess of 500° C. An example being the deposition ofN+polysilicon which is typically deposited at temperatures in excess of500° C.

In addition, since the cured films contain considerable silicon-oxygenfunctionality, they are resistant to oxygen RIE etching. However, thesesame coatings are readily etchable using CF₄ /O₂ (20:80) orchemical/mechanical polishing. A comparison of the etch rate of thethermostable composite with other commonly used semiconductor films andmasks can be seen in FIG. 3. It should be noted that the etch rate ofthe thermostable composite exceeds that for SiN_(x) when exposed to amixture of 20/80 (CF₄ /O₂) but it is significantly lower than the etchrate of SiN_(x) in 100% O₂.

EXAMPLE 1

Perylene dianhydride (20.2 g, 0.0515 mol) is introduced into a 250 mLpolyethylene bottle and suspended in 100 mL dry NMP. Aminopropyltriethoxysilane (45 mL, 0.1914 mol) is added to the mixture and thesolution is stirred overnight. The foamy product settles into a deepred/brown solution, which can be warmed to 80° C. to insure uniformity.The resulting solution formed good spin on films when high torque (4000rpm) and low spin times (10-15 sec.) were employed. After heating, theoligomeric composition favored good planarization and gap filling, andthermal testing demonstrated high temperature stability of thecomposite, see FIG. 2. The cured films contained considerablesilicon-oxygen functionality and were resistant to oxygen RIE. However,the coatings were readily etchable using CF₄ /O₂ (20:80) RIE conditions.

EXAMPLE 2

Ten milliliters of aminopropyltriethoxysilane and 3.92g of perylenedianhydride were added to 20 mLs of NMP to form a solution. Thissolution was spin applied to silicon wafers, metalized wafers andpatterned wafers at 500 RPMs for 7 seconds, 1000 RPMs for 7 seconds, and3000 RPMs for 30 seconds. Thereafter it was baked at105°-115°-125°-135°-150°-150°-1150° C. for a total of 10 minutes andcured at 500° C. for 30 minutes in nitrogen atmosphere. The resultingfilms on the respective wafers were tested and the results obtainedwere:

etch rate at 20:80 CF₄ /O₂ was approximately 5000 A°/min and the etchrate at 100% O₂ was approximately 33 A°/min

1 μthick post 500° C.;

Film stress was 30 mPa/Tensile;

Chemical Mechanical Polishing rates varied from 2,500 A°/min. to 1300 A°/min. depending on the slurry selected whereas the CMP rate of SiN_(x)under the same conditions was approximately 40°-50 A°/min.

EXAMPLE 3

Perylene dianhydride, aminopropyltriethoxysilane (A1100), NMP and waterwere combined in a plastic bottle, in the amounts indicated in the chartbelow, and then mixed on a roller mill overnight. Thereafter, thesolutions were applied to a semiconductor substrate via spin-ontechniques, heated as in Example 2, and cured at 563° C. for 30 minutesin a N₂ atmosphere. The results were as follows:

    __________________________________________________________________________       Perylene                                                                   Trial                                                                            Dianhydride                                                                          A1100                                                                              H.sub.2 O                                                                         NMP Results                                                __________________________________________________________________________    1  3.92 g 9.40 mL                                                                            --  20 mL                                                                             Solids remain in solution but films                       (0.010 (0.040       prepared are good                                         moles) moles)                                                              2  3.92 g 10.0 mL                                                                            --  20 mL                                                                             Dewets appear in cured films                                     (0.0425                                                                       moles)                                                              3  3.93 g 10.0 mL                                                                            2 drops                                                                           20 mL                                                                             Dewets, worse than those of #2, appear in                                     the cured films                                        4  3.92 g 10.0 mL                                                                            --  20 mL                                                                             Dewets, worse than those of #2, appear in                                     the cured films                                        5  3.92 g +                                                                             10.0 mL                                                                            --  20 mL                                                                             Good films are produced upon cure                         excess                                                                     6  30.3 g 72.6 g                                                                             --  200 mL                                                                            Dewets apear in cured film                                (0.773 (0.3092                                                                moles) moles)                                                              7  19.6 g 47 mL                                                                              --  100 mL                                                                            Good films produced upon cure                             (0.050 (0.2000                                                                moles) moles)                                                              8  21.0 g 50.35 mL                                                                           --  107 mL                                                                            The cured film possesses some dewets and                  (0.05357                                                                             (0.2143      the polymer puckered on cure                              moles) moles)                                                              __________________________________________________________________________

EXAMPLE 4

A1100 is placed in a plastic bottle along with a specified amount ofwater; it is then equilibrated by placing the bottle on a roller millfor approximately 1 hour. Thereafter, perylene dianhydride, NMP and theequilibrated aminopropyltriethoxysilane (A1100) were combined in aseparate plastic bottle and mixed on a roller mill overnight. Thesolution was applied to a substrate in accordance with the conditionsstated above in respect to Example 3. The component ratios used in theindividual runs and the results of these trials are indicated in thechart below.

    __________________________________________________________________________                          Tetrafluoro-                                               Perylene           phthalic                                                Trial                                                                            Dianhydride                                                                          A1100                                                                             H.sub.2 O                                                                        NMP  Acid   Results                                          __________________________________________________________________________    1  19.6 g 47 mL                                                                             47 μl                                                                         100 mL                                                                             --     Good films were                                     (0.050 (0.200             formed upon curing                                  moles) moles)                                                              2  5.0 g  12 mL                                                                             6 μl                                                                          25.5 mL                                                                            --     The film does not                                   (0.01275                                                                             (0.0510            dewet although it                                   moles) 8                  hazes upon the initial                                     moles)             heating                                          3  5.0 g  12 mL                                                                             12 μl                                                                         25.5 mL                                                                            --     The film does not                                                             dewet although it                                                             hazes upon the initial                                                        heating                                          4  5.0 g  12 mL                                                                             24 μl                                                                         25.5 mL                                                                            --     The film does not                                                             dewet although it                                                             hazes upon the initial                                                        heating                                          5  19.6 g 47 mL                                                                             12 μl                                                                         100 mL                                                                             0.56 g Reduced dewets                                   __________________________________________________________________________

EXAMPLE 5

A solution is formed by mixing 3.02 g (0.007704 moles) of perylenedianhydride, 15.4 ml NMP and 7.24 ml (0.03081 moles) of A1100. Thesolution is heated to a temperature of 50° C. for a period of 12 hoursand thereafter applied to a semiconductor substrate and cured inaccordance with the techniques of Example #3 above. Good films wereproduced under this process.

EXAMPLE 6

A solution was similarly formed using the same material and ratios asExample 5 above except that the amount of (aminoethylaminomethyl)phenethyltrimethoxysilane (A0698) added was 6 ml. Unreacted anhydrideremained after having been on the roller mill overnight. Then 4.7 ml ofA1100 and 5 ml NMP were added to the contents of the plastic bottlethereby forming a good solution, which was stable for approximately 5days or more.

EXAMPLE 7

The compound of formula IV (produced from the mixture of perylenedianhydride and A1100 in NMP) is spun onto the surface of a siliconbased wafer. The compound is applied at static apply for 7 seconds, 500RPM for 7 seconds, 1000 RPM for 7 seconds and 3000 RPM for 15 seconds.The wafer is then heated in air in a ramped fashion(105°-115°-125°-135°-150°-150°-150°-1150°) to 150° C. over 10 minutesfollowed by heating in a N₂ atmosphere furnace for 30 minutes at 620° C.Thereafter, a positive photoresist is applied to the wafer and baked.Selected portions of the photoresist are then exposed on a G-linestepper. KOH is used to develop the photoresist by exposing the desiredportions of the compound of formula V. These exposed portions are thenetched in 80:20 O₂ /CF₄ for 4 minutes thereby forming a patterned layerof the compound of formula V. The remaining photoresist is then strippedby application of N-butyl acetate and isopropyl alcohol (NBA/IPA 3minutes/3 minutes). Thereafter, a 950 Å layer of polysilicon isdeposited upon the wafer at 590° C. over 10 minutes.

While the invention has been particularly shown and described withreference to preferred embodiments thereof, it will be understood bythose skilled in the art that other changes in the form and details maybe made therein without departing from the spirit and scope of theinvention.

What is claimed is:
 1. A thermostable composite consisting essentiallyof a perylene diimide having the structure ##STR5## and apoly(silsesquioxane), said thermostable composite produced by theprocess of:(a) combining perylene dianhydride and a primary or secondaryaminoalkyl-alkoxysilane in a solvent in a molar ratio of about 1:4; (b)mixing at a temperature between 15° C. and 80° C. for 4 to 20 hours; (c)heating under conditions such that a mechanically stable solid isformed; and (d) curing at 500° to 620° C. under nitrogen for about 30minutes.
 2. A thermostable composite produced by the process of claim 1comprising:a combining perylene dianhydride and a primary or secondaryaminosilane in a solvent in a molar ratio of about 1:4; (b) mixing at atemperature between 15° and 80° C. for 4 to 20 hours; (c) setting byheating to final temperature of about 150° C. over the course of 10 to30 minutes; and (d) curing at 500° to 620° C. under nitrogen for about30 minutes.
 3. The composite according to claim 1 further characterizedin that said composite exhibits less than 1% weight loss upon exposureto temperatures from 500° to 600° C. for one hour.
 4. The compositeaccording to claim 1 further characterized in that said compositeexhibits an RIE etch rate more than 4 time that of silicon nitride in20/80 CF₄ /O₂ and less than 1/4 that of silicon nitride in pure O₂. 5.The composite of claim 1 wherein said composite is a result of thereaction of:perylene dianhydride; and an aminosilane having a formulaof: ##STR6## wherein R is selected from the group consisting of H, alkyland alkylamine; X is selected from the group consisting of alkyl of atleast 2 carbons and aryl; Z is alkyl or a direct bond; Y is selectedfrom the group consisting of alkoxy, halo and silazane.
 6. The compositeof claim 5 wherein the aminosilane is an aminoalkyltrialkoxysilane. 7.The composite of claim 6 wherein the aminoalkyltrialkoxysilane isaminopropyltriethoxysilane.
 8. A thermostable composite, consistingessentially of:(a) a perylene diimide having the structure ##STR7## and(b) a poly(silsesquioxane).
 9. A thermostable composite according toclaim 8 wherein said composite comprises about four silicon atoms in thepoly(silsesquioxane) for each molecule of perylene diimide.
 10. Athermostable composite according to claim 8 wherein said compositecomprises at least four silicon atoms in the poly(silsesquioxane) foreach molecule of perylene diimide.